This invention relates generally to thin film batteries, and more particularly to the production of lithium cathodes of thin film, rechargeable lithium ion batteries.
Conventional, canister type batteries today include toxic materials such as cadmium, mercury, lead and acid electrolytes. These chemicals are presently facing governmental regulations or bans as manufacturing materials, thus limiting their use as battery components. Another problem associated with these battery materials is that the amount of energy stored and delivered by these batteries is directly related to the size and weight of the active components used therein. Large batteries, such as those found in automobiles, produce large amounts of current but have very low energy densities (Watts hours per liter) and specific energies (Watt hours per gram). As such, they require lengthy recharge times which render them impractical for many uses.
To address the need for higher energy densities and specific energies, the battery industry has been moving towards lithium based batteries. The major focus of the battery industry has been on liquid and polymer electrolyte systems. However, these systems have inherent safety problems because of the volatile nature of the electrolyte solvents. Furthermore, these types of batteries have a relatively high ratio of inert material components, such as the current collector, separator, and substrate, relative to the active energy storage materials used for the anode and cathode. In addition, their relatively high internal impedance results in low rate capability (watts/kilogram) which renders them impractical for many applications.
Thin film lithium batteries have been produced which have a stacked configuration of films commencing with an inert ceramic substrate upon which a cathode current collector and cathode are mounted. A solid state electrolyte is deposited upon the cathode, an anode in turn deposited upon the electrolyte, and an anode current collector mounted upon the anode. Typically, a protective coating is applied over the entire cell. Lithium batteries of this type are describe in detail in U.S. Pat. Nos. 5,569,520 and 5,597,660, the disclosures of which are specifically incorporated herein. However, the lithiated cathode material of these batteries have a (003) alignment of the lithium cells, as shown in FIG. 1, which creates a high internal cell resistance resulting in large capacity losses.
Thin film batteries have also been produced by forming active cathode materials through chemical vapor deposition techniques. In the past, chemical vapor deposition cathodes have been manufactured in extremely low pressure environments, within a range of 1-100 torr. The requirements of this extremely low pressure environment greatly increases the cost of production and greatly reduces the feasibility of producing commercially viable products as a result of the difficulty in controlling such. Furthermore, this type of chemical vapor deposition is typically carried out by heating a precursor solution to cause evaporation of the solution to a gas phase so that it may be carried off to a deposition location through a stream of non-reactive gas, such as argon. The heating of the precursor for an extended time period can cause the solution to decompose and therefor become unworkable. Furthermore, the high temperature and low pressure of the system requires extensive heating of the transport lines conveying the solution to prevent the evaporated solution from condensing between the heating location and the deposition location, thus further increasing the cost and the complications involved in production. Lastly, heretofore the solutions utilized in chemical vapor deposition of lithiated material were highly reactive and sensitive to air, hence the need for the argon gas environment. The resulting product was also amorphous and inhomogeneous thereby greatly restricting its use in thin film battery applications.
Recently, it has been discovered that the annealing of lithiated cathode materials on a substrate under proper conditions results in batteries having significantly enhanced performances, for the annealing causes the lithiated material to crystalize. This crystallized material has a hexagonal layered structure in which alternating planes containing Li and Co ions are separated by close packed oxygen layers. It has been discovered that LiCoO2 films deposited onto an alumina substrate by magnetron sputtering and crystallized by annealing at 700xc2x0 C. exhibit a high degree of preferred orientation or texturing with the layers of the oxygen, cobalt and lithium oriented generally normal to the substrate, as illustrated by the (101) plane shown in FIG. 2. This orientation is preferred as it provides for high lithium ion diffusion through the cathode since the lithium planes are aligned parallel to the direction of current flow. It is believed that the preferred orientation is formed because the extreme heating during annealing creates a large volume strain energy oriented generally parallel to the underlying rigid substrate surface. As the crystals form they naturally grow in the direction of the least energy strain, as such the annealing process and its resulting volume strain energy promotes crystal growth in a direction generally normal to the underlying substrate surface, which also is the preferred orientation for ion diffusion through the crystal.
In the past, with an annealing temperature below 600xc2x0 C. the lithium material has no significant change in the microstructure, and thus the lithium orientation remains amorphous, as taught in Characterization of Thin-Film Rechargeable Lithium Batteries With Lithium Cobalt Oxide Cathodes, in the Journal of The Electrochemical Society, Vol. 143, No 10, by B.Wang, J. B. Bates, F. X. Hart, B. C. Sales, R. A. Zuhr and J. D. Robertson. This amorphous state restricts lithium ion diffusion through the layers of oxygen and cobalt, and therefore creates a high internal cell resistance resulting in large capacity losses.
Hence, in order to anneal the lithiated cathode material to the most efficient orientation it was believed that the cathode had to be bonded to a rigid substrate and heated to nearly 700xc2x0 C. for an extended period of time.
It thus is seen that a need remains for a method of producing a cathode for use in high performance rechargeable, thin film lithium battery without the need for an extemely low pressure system and without annealing the cathode. Accordingly, it is to the provision of such that the present invention is primarily directed.
In a preferred form of the invention, a method of producing a layer of LiCoO2 comprises the steps of providing a solution having a mixture of Li(TMHD) and Co(acac)3 dissolved in an organic solvent, processing the solution to form a mist, heating the solution mist to a vapor state, and depositing the vapor on a substrate.